Cable telecommunications systems have been known for a number of years and are currently gaining in popularity and coverage for the distribution of television programming, telephone service and networking of computers such as providing Internet access since they can carry many signals over a wide bandwidth with little, if any interference or distortion, particularly as data transmission rates have increased to accommodate high definition television, increased volume of digital communication and the like. By the same token, since these communications are intended to be confined within the cable system, the increased bandwidth required for such communications need not be allocated from the available bandwidth for other communications such as radio, navigation, GPS, emergency communications and the like which must be transmitted as free-space electromagnetic waves through the environment. However, flaws in cable shielding in cable telecommunication systems can allow signal egress which can potentially interfere with broadcast communications and potentially cause hazards. Reciprocally, flaws in cable shielding can permit signal ingress into the cable from the environment and degrade or interfere with the signal being carried by the cable telecommunication system. Therefore, such flaws must be quickly discovered and remedied as they occur due to weather, mechanical damage or the like.
Detection of cable shielding flaws is generally achieved through detection of a signal carried by the cable transmission system that has leaked into the environment, essentially by being broadcast from the shielding flaw. Detection of a signal that has leaked or egressed from a cable flaw is generally performed in two stages: first, by a receiver in a mobile vehicle driven in the general vicinity of installed cables that associates a received signal with a location of the mobile vehicle using a global positioning system (GPS) receiver which thus reports a general location of a shielding flaw and, second, by a hand-held instrument that can allow repair personnel to follow increasing egress signal strength to the exact location of the shielding flaw so that repairs and/or maintenance can be carried out.
Of course, such detections must be carried out in an environment in which noise as well as broadcast signals will also be present in the same frequency bands. Accordingly, a problem with all such systems is to identify a received signal as one originating in the cable telecommunications system. For example, in systems in which at least some television programming is carried as an amplitude modulated signal such as an amplitude modulated NTSC signal (as was historically the case for all television programming), leakage detection equipment could monitor the television channel visual carrier frequency. However, in recent years, to increase the amount of information that can be transmitted over a cable system and to support additional data services such as telecommunications and computer networking (e.g. Internet access), more efficient digital communications using a form of quadrature amplitude modulation (QAM) are being increasingly employed to the virtual exclusion of analog communications over broadband cable communication systems. The resulting signal spectrum on the cable system is of substantially constant amplitude (since the carrier is suppressed in each of the many closely spaced frequency bands within the broadband spectrum) and thus closely resembles white broadband background noise that may contain any number of free-space broadcast signals. Further, the signal-to-noise ratio (SNR) of an egress signal, when present in such background noise may be −25 dB or less relative to the background noise. Therefore, it has proven extremely difficult to distinguish and authenticate a signal representing signal egress from a broadband communication system transporting only QAM signaling from the background noise at any given location.
Therefore, prior egress signal detectors for use in a QAM environment have employed active addition of marker signals to facilitate detection and authentication of a detected signal as an egress signal from a broadband communication system even though use of such a marker signal generally requires that a finite portion of the available bandwidth be dedicated to the marker signal. Further, use of a marker signal while seeking to minimize the bandwidth allocated to it implies a need for accurate control of both frequency and signal strength of the marker signal in avoiding interference with signals in adjacent frequency bands carrying information in the cable system as is provided in U.S. patent application Ser. No. 13/080,715 by John Murphy et al. now U.S. Pat. No. 8,749,248, issued Jun. 10, 2014, which is hereby fully incorporated by reference and provides a system for successfully managing use of a marker signal for egress signal detection in a QAM environment in order to minimize both the amount of bandwidth dedicated to the marker signal and interference of the marker with information signals carried by the system. This system also provides the facility of distinguishing between broadband communication systems that may be constructed in close physical proximity to each other (often referred to as systems which are built over each other or “over-built”) although some complexity in providing different marker signals in the respective systems has been encountered.